Waiting is a shared experience, and people respond strongly to shared experiences. As Marketwatch puts it, "time wasted is camaraderie gained."

Shoppers reinforce each other's thoughts on making the right buying decision when they stand in a crowd. It's a concept called "social proof."

Retailers like long lines because it creates attention. This holds true even after the initial hype dies down-people will still be eager to buy a product if they saw all the attention it initially generated.